insertion function example

insertat.c

#include <stdio.h>


/*	This insertAt function takes an index to insert at "i",
 *	a number to insert into the integer array "num", a pointer
 *	to the current number of used spaces in the array "numUsed",
 *	and a pointer to the array "a".
 *	
 *	This assumes that the array is big enough to insert a new element
 *	for the sake of simplifying the example so it doesn't check against
 *	the number of allocated elements.
 *
 *	The variable j never goes below zero as long as the index i
 *	we are trying to add at is greater than or equal to zero (which it
 *	certainly should be).  This is because the for loop's condition
 *	is that it be greater than i and the only sane values for i are greater
 *	than or equal to 0.  The loop copies all the values from index j-1 
 *	to index j and once it sees that j is equal to i it exits the loop
 *	so it never has a chance to access the value at i-1.  Once
 *	j is equal to i we only have to set a[j] (which is the same as a[i]
 *	as at this point i == j since that was essentially our exit condition)
 *	to num.  We then remember to increment numUsed and return.
 */
void insertAt(size_t i, int num, size_t *numUsed, int a[]) {
	size_t j;
	for (j = *numUsed; j > i; j--) {
		a[j] = a[j-1];
	}
	a[j] = num;
	(*numUsed)++;
	return;
}


int main() {

	// Make a new array with 20 allocated
	// set the first ten indices to 10 through 19
	int a[20];
	size_t i;
	for (i = 0; i < 10; i++) {
		a[i] = i+10;
	}
	// Only the first ten indices are in use.
	size_t numUsed = 10;

	// Add a few numbers to the array...
	insertAt(0, 123, &numUsed, a);
	insertAt(5, 456, &numUsed, a);
	insertAt(9, 789, &numUsed, a);

	for (i = 0; i < numUsed; i++)
		printf("%d\t%d\n", i, a[i]);
	
	return 0;
}